This invention relates to an aircraft door stop, and more particularly to a door stop pin assembly for alleviating galling, overstressing, and misalignment problems.
As a result of normal assembly operations for door stop fittings, there is a small angular misalignment between pairs of mating stop fittings, i.e., the axis of the stop pad is not parallel to the axis of the stop pin. The problems related to this geometric condition depends upon what type of stop pin is being considered: (a) for a stop pin with a large head radius, it means that the "point" of contact will not be located on the axis of the stop pin. There will still be a "point" contact, and if stresses are high enough, galling will occur; (b) for a stop pin with a flat head, the flat surface of the head will not make full contact, but instead will make contact at some point along the bottom edge. Again, if the stresses are high enough, galling will occur.
Galling is an undesirable development and is a bearing stress problem. Advanced stages of galling can result in excessive resistance to the lateral sliding of the stop pin on the stop pad. This large lateral load contributes toward fatigue failure of door stop fittings. Both types of existing pins, (a) and (b) above, can lead toward fatigue failure of the door stop fittings due to galled surfaces.
In the patent literature, U.S. Pat. No. 3,187,372 to Parsons, issued Jun. 8, 1965, discloses a self-aligning pin to accommodate the change in geometry of the stop assembly, in contrast to the present invention which provides means for transferring high loads between two elements of structure that may not be disposed precisely ninety degrees with respect to each other. The present preferred embodiment of the invention accommodates relative motion between the self-aligning stop pin assembly and its mating stop pad.
Returning to the stop pin discussed in (a) hereinabove, in sum it can be stated that angular misalignment of the stop fittings has very little, if any, effect upon the development of galling. Even with perfect alignment, there will be a "point" contact between the pin and the pad. In (b) discussed hereinabove, a different situation exists In this case, the misalignment directly contributes to the development of galling by initiating a "point" contact condition.
Having understood the preceding discussion, there is one other aspect requiring explanation regarding the misalignment of the door stop fittings. Since the stop pad surface will usually be abnormal to the axis of the stop pin, this means that on occasion the tilt of the stop Pad will be such that in order for the stop pin to slide in its preferred lateral direction, the pin will need to slide "uphill." To slide uphill requires that the pin accept more axial compressive load. The tendency of the pin is to resist this "uphill" movement and instead of sliding, the pin transfers its lateral resistance load to its own fitting. In a situation such as this, the self-aligning device, according to the preferred embodiment of the present invention, does very little to reduce the magnitude of the lateral loads exerted upon the pin fitting; however, its redeeming feature is that it will help inhibit the development of galling, and thus prevent the increase in magnitude of the lateral loads acting on the pin fitting that would be associated with advanced galling.
The majority of Prior attempts to reduce the lateral loads transferred to the door stop fittings have been related to reducing the sliding friction forces between the pin and the pad utilizing various lubricants. In spite of these efforts, problems related to galling still occur.